With increasing requirements for high-speed data transmission, LTE (long-term evolution) becomes one of the most competitive wireless transmission technologies. A user equipment (UE) may access to an LTE network only after a process including cell search, acquiring system information of cell and random access. The major purpose of the cell search includes: (1) synchronizing with the cell in terms of frequency and symbol; (2) acquiring system frame timing, i.e., a starting position of a downlink frame; and (3) determining a physical layer cell identity (PCI) of the cell. The UE performs the cell search on start-up, however, in order to support the mobility, the UE may continuously search for a neighboring cell, perform synchronization and estimate the quality of the received signal for the neighboring cell, so as to determine whether to perform handover or cell reselection.
With the increasing requirements for data transmission, a critical way for improving overall performance of the system is to increase transmission bandwidth and improve spectrum utilization ratio. Under this background, more and more operators focus on using unauthorized frequency bands, and consider taking the unauthorized frequency bands as a supplementary for existing LTE authorized frequency bands to improve quality of service for users.
Communication on the unauthorized frequency band for a cell and communication for other systems co-exist and share the frequency spectrum resource. Therefore, to a certain extent, the signal of the cell must retreat due to other systems having a higher priority (such as radar), or the cell may operate in a fixed time period after negotiating with other systems (such as WiFi). If the cell search and synchronization take a long time, it is not beneficial for initial access of the UE, cell reselection and handover, which results in limitation for the usage of the unauthorized frequency band. Therefore, it is desired that the UE can perform search and synchronize rapidly with respect to a cell on the unauthorized frequency band.